1. Field of the Invention
The present invention relates to a radiation detector detecting radiations, and more particularly, to a radiation detector, including an operation confirming unit using an optical pulse, which is continuously usable for a long time.
2. Related Art
In a conventional radiation detector, a radiation source called a bug source is provided in or outside the radiation detector. A certain amount of radiations is emitted from the bug source into a radiation detector in the radiation detector, and a response of the radiation detector to the radiations incident on the radiation detector is evaluated in order to confirm the operational integrity of the radiation detector in use. However, it is complicated to treat the radiation source used as the bug source and it is difficult to confirm the operation of the radiation detector over the entire measurement range.
In some radiation detectors which do not use the bug source and use a light-sensitive radiation sensor, an optical pulses emitted from optical pulse generator is provided in the radiation detector, the radiation sensor is irradiated with the optical pulse, and the response of the radiation detector to the optical pulses is verified.
Japanese Unexamined Patent Application Publication No. 6-72930 discloses an operation confirming unit for a radiation detector, using optical pulses. A radiation sensor in the radiation detector is irradiated with an optical pulse to detect any abnormality in the bias voltage on the basis of a response from the radiation sensor. The operation confirming unit detects any abnormality in the bias voltage in accordance with the fact whether the response of the radiation detector to the optical pulse is within a predetermined range or not.
Japanese Unexamined Patent Application Publication No. 2006-84345 discloses a method of confirming the operational integrity of a radiation detector by using optical pulses. An optical pulse emitting unit is used as radiation means for calibration. Data concerning the pulse height spectrum of the radiation detector irradiated with an optical pulse is analyzed to evaluate the operational integrity of the radiation detector.
However, since the radiation means for calibration does not always offer a constant amount of output in the radiation detectors disclosed in both the Japanese Unexamined Patent Application Publication No. 6-72930 and No. 2006-84345, it is not possible to constantly evaluate or confirm the integrity of the radiation detector. This is because (1) continuous use of the optical pulse emitting unit for a long time varies the amount of light emission and (2) a difference between the time response characteristics of the light emission and those of the radiation possibly damages the function of the radiation detector, which detects the radiations.
Particularly, if the radiation detector has a function of correcting a dead time, the time response characteristics of the light emission which are different from those of the radiation can increase a difference based on the function of correcting the dead time.
Accordingly, the operation confirming unit using optical pulses in the related art causes no problem when it is used as a temporal operation confirming unit. However, when the operation confirming unit using optical pulses in the related art is constantly used, the radiation detector cannot possibly measure any variation with age of the radiations to be measured correctly. Consequently, it is difficult to continue to constantly use the operation confirming function while keeping the reliability of the radiation detector as a radiation measuring apparatus.